The present invention relates to digital wireless communications systems and more particularly, to a method and apparatus for a carrier frequency control in such systems.
Over the past few years, various wireless architectures have been developed in response to user demands for systems that can offer high-rate data communications. Recently for example, broadband wireless access (BWA) systems have become of interest to the wireless industry for their ability to supply high speed multimedia data services.
In a wireless communication system such as a BWA network, it is common to speak of a point-to-multipoint architecture. In this architecture, a base transceiver station (BTS), is positioned at the center of a service area (normally called a cell) and communicates over the air with multiple customer premises equipment (CPE) units located within the same cell. The BTS is usually fixed and designed for servicing all of the CPE units present in the cell. The CPE units can be designed to have fixed locations within the cell or alternatively be portable and free to roam.
Typically, the direction of communication from the BTS to the CPE units is called the downstream direction and the reverse direction of communication is referred to as the upstream direction. In order to send separate downstream data streams to multiple CPE units at the same time, the data is usually multiplexed. There is presently various multiplexing methods used for downstream communications. For example, downstream data is often transmitted by frequency division multiple access (FDMA) by assigning a distinct downstream carrier frequency to each CPE unit.
More commonly however, downstream data destined for several CPE units is multiplexed in time and transmitted at a common carrier frequency. This can also apply to upstream communications with upstream data originating from several CPE units multiplexed in time and transmitted at another common carrier frequency. A cellular system that has these characteristics is usually referred to as a time-division multiple access (TDMA) system.
In order to multiplex several data streams into a single TDMA stream, the time structure of an upstream communications link is divided into scheduling periods having a fixed number of time slots per scheduling period. The CPE units can only access the upstream link if an opportunity is granted in a scheduling period by the BTS. When an opportunity is given to a particular CPE unit to transmit on the upstream link, the CPE unit initially notifies the BTS of the amount of upstream bandwidth it requires for its transmission. This bandwidth on-demand process causes upstream transmissions to occur in bursts. The BTS receiver must be designed to operate rapidly and on a burst-by-burst basis for upstream transmissions to efficiently service each CPE unit present in the cell.
The upstream and downstream data is transmitted in dedicated high-frequency radio channels at radio frequencies (RF) which, in today""s systems, typically range from a few gigahertz (GHz) up to 50 GHz. At each of the BTS and CPE units, local oscillators are used to generate carrier signals that can operate at these high frequencies so that data can be transmitted and/or received in the RF frequency range of interest.
An undesirable characteristic associated with the vast majority of conventional oscillators is that the carrier signals generated can be subject to frequency drifts which arise as a result of temperature changes and aging effects. The severity of the frequency drifts is directly proportional to the oscillating frequency and thus increases as the oscillating frequency increases. In wireless systems, because of the high frequency of operation, the frequency drifts introduced can be quite large, commonly in the order of hundreds of Kilohertz (KHz). Such large frequency drifts are difficult to track and can seriously affect the ability with which data can be reliably received.
A key aspect in maintaining a reliable radio link between CPE units and a BTS in a wireless system is the ability to counteract these carrier frequency variations. One possible way to address this problem is to increase the frequency stability of the local oscillators used. Unfortunately, this considerably increases design complexity in each of the BTS and CPE units and is often not economically sound.
Carrier recovery loops (CRLs) are commonly used in wireless systems to track and compensate small carrier frequency drifts. However, conventional CRL loops are not suited for large carrier frequency variations which, as noted above, are inevitable at high carrier frequencies. As is well known, large frequency drifts in a carrier can easily cause upstream and downstream transmissions to fall outside the carrier acquisition range of a conventional CRL loop and potentially result in link failures.
In a traditional point-to-point wireless system, it is known to use CRL loops equipped with frequency sweep capabilities (hereinafter also referred to as a frequency sweep CRL loop). The use of a frequency sweep control in a CRL loop is highly desirable because it increases the CRL loop carrier acquisition range and allows tracking of larger carrier frequency drifts.
However, despite this important advantage, the use of a frequency sweep mechanism in a CRL loop unfortunately reduces the speed at which the CRL loop can perform carrier acquisitions. In some receivers such as CPE receivers for example, a slow carrier acquisition speed may be tolerable because the carrier signal to acquire is continuous and thus available for a complete acquisition. In other types of receivers however, where the carrier signal received is instead formed of discontinuous bursts, a frequency sweep CRL loop may simply not be fast enough.
In a BTS receiver for example, a frequency sweep CRL loop would not be capable of providing carrier acquisitions sufficiently rapidly for receiving upstream signals transmitted in bursts from different CPE units. In current wireless systems, upstream signals are typically transmitted in bursts of varying sizes and duration where each burst is from a different CPE unit. For each signal burst of upstream data received, the BTS receiver only has available a very short period of time to perform a complete carrier acquisition before it can begin to receive the data transmitted. This is particularly true for upstream bursts of a short duration where a short acquisition time is critical for reliable data reception.
Because of their slow carrier acquisition speed, frequency sweep CRL loops simply cannot perform carrier acquisitions sufficiently quickly. Therefore, it would be desirable to provide a carrier frequency compensation scheme at the BTS which is capable of fast burst-to-burst carrier acquisitions and capable of counteracting potentially large carrier frequency drifts in each signal burst received from every CPE unit present in a cell.
The invention provides a carrier frequency control method and apparatus for efficiently controlling in a wireless system the carrier frequency of a received signal transmitted over a radio link to counteract carrier frequency drifts in the received signal and maintain link connectivity.
In a preferred embodiment, the invention provides upstream and downstream carrier frequency control in a broadband wireless access (BWA) time division multiplex access (TDMA) system formed of a base transceiver station (BTS) and multiple customer premises equipment (CPE) units.
The upstream carrier frequency control in each CPE unit is based on estimates of downstream carrier variations in the downstream carrier signal received therein. More specifically, after a successful acquisition of the downstream carrier signal received, each CPE unit estimates a downstream carrier frequency offset between the downstream carrier signal frequency received and an expected downstream carrier signal frequency. Based on the downstream carrier frequency offset estimated, each CPE unit preemptively adjusts its respective upstream carrier frequency. By preemptively adjusting the CPE upstream carrier frequency at each CPE unit, variations in the upstream carrier frequency subsequently appearing in the upstream data as a result of frequency conversions to and from radio frequencies are efficiently counteracted.
According to the invention, any residual offset not cancelled by preemptive offsetting of the upstream carrier frequency can be advantageously handled by a conventional carrier recovery loop (CRL) at the BTS.
Another advantage is that the preemptive offsetting of the upstream carrier frequency at each CPE unit ensures that the BTS can in fact receive upstream signals more reliably. This in turn considerably improves upstream carrier acquisition performance at the BTS.
Similarly to the upstream carrier frequency control, the downstream carrier frequency control at the BTS is also based on estimates of downstream carrier variations in the downstream carrier signal received at the CPE units. In the preferred embodiment, the BTS periodically polls all active CPE units to obtain downstream frequency offset estimates. In each polling period, the BTS calculates a frequency correction offset based on the estimates received and adjusts its downstream carrier frequency accordingly. The downstream carrier frequency adjustment advantageously compensates carrier frequency variations caused locally within the BTS and considerably improves downstream carrier acquisition performance at each CPE unit.